execute dxdiag found in winnt\system32 subdirectory. don't know the english word for the tab you should switch to, since i think you are a german user try "Weitere Hilfe" and click on "Außer Kraft setzen..".
Now change to "Wert für Außerkraftsetzung" and put in your desired refresh rate. note: refresh rate is set for ALL resolutions so check if your monitor can handle them.